Multilingual embeddings: setup guide + cross-lingual eval

[ahmetvural79]

Cross-lingual retrieval (ask in one language, hit a document in another) comes from the embedder, not keyword search. docs/PROVIDERS.md covers the what but not an end-to-end setup + an eval that proves it.

Scope

• step-by-step OpenAI and Ollama embedder setup
• a small mixed-language fixture brain
• an eval asserting a query in language A retrieves a source in language B
• note PGLite has no stemmer/unaccent (vector signal carries morphology)